The best way to increase the rust resistance of a steel outdoor locking mailbox is to use a zinc alloy steel. However, this is more expensive than normal cold rolled steel.
Another method of increasing the rust resistance in your curbside locking mailbox is to use two coats of paint, a primer coat and a powder coat.
Most steel locking mailboxes today on the market do not use a primer coat, they only use a powder coat over a galvanized steel. While the galvanization does provide some rust protection, it can still rust at the cut ends and the welds. If a zinc alloy steel is not used, then the next best thing is to use a high zinc content primer coat before applying the powder coat.
Most outdoor steel fencing (decorative type fencing) uses the double painting method. Most mailbox manufacturers should take a page from the book of the fence makers and do a double coat.
The MailCase solves the problem by using the rust resistant high zinc-steel alloy.